1. Field of the invention
The invention relates to multistep hydrocarbon conversion processes which produce ethers from an alcohol feed stream and a feed stream comprising light paraffins. The invention more directly relates to integrated processes which utilize paraffin isomerization and paraffin dehydrogenation steps to produce isoolefins for subsequent reaction with an alcohol in an etherification zone. The invention is specifically related to the circulation of olefin containing streams between the dehydrogenation and the isomerization zone.
2. Prior Art
Etherification processes are utilized to produce large amounts of gasoline boiling range ethers for use as anti-knock compounds in lead-free gasoline. Ethers may also be produced as a means of making pure isoolefins. For instance, plans have been announced to produce pure isobutene for the manufacture of polyisobutylenes and tert-butyl-phenol by cracking methyl tertiary butyl ether (MTBE), with the methanol being recycled. The predominant etherification process involves the reaction of an alcohol with an isoolefin, with C.sub.4 and C.sub.5 isoolefins being the preferred feedstock. As the supply of C.sub.4 and C.sub.5 isoolefins is rather limited since they are mainly produced as by-products of fluidized catalytic cracking or steam cracking, it is often necessary to produce the isoolefins by the sequential steps of isomerization and dehydrogenation.
The production of ethers by the reaction of an isoolefin with an alcohol is well known and is practiced commercially. This highly selective reaction is also used to remove isoolefins, especially isobutylene, from mixed hydrocarbon streams such as the C.sub.4 streams produced in steam cracking plants which produce ethylene. Increased attention has been focused on ether production due to the rapidly increasing demand for lead-free octane boosters for gasoline such as MTBE. A detailed description of processes, including catalysts, processing conditions and product recovery, for the production of MTBE from isobutylene and methanol are provided in U.S. Pat. Nos. 2,720,547 and 4,219,678 and in an article at page 35 of the June 25, 1979 edition of Chemical and Engineering News. The preferred process is described in a paper presented at the American Institute of Chemical Engineers 85th National Meeting on June 4-8, 1978 by F. Obenaus, et al. Descriptions of integrated processes, including those which utilize butane isomerization and/or butane dehydrogenation, are found in U.S. Pat. Nos. 3,726,942, 4,118,425, 4,252,541 and 4,329,516.
Except for U.S. Pat. No. 3,726,942, all of the process arrangements include an isomerization step ahead of and isolated from a dehydrogenation and further processing steps. Since the isomerization of relatively short chain paraffins yields a fixed ratio of isoparaffin to paraffin, it is advantageous to recycle unconverted paraffins back to the isomerization reactor. Recycling of unconverted normal paraffins has usually been performed by separating the normal paraffins from the effluent of the isomerization reactor.
The above schemes suffer two drawbacks. First, the feed to the isomerization process must consist essentially of normal paraffins and isoparaffins. This makes the process adaptable only to such feeds or requires additional processing facilities to remove other components. In addition, process streams containing unbranched paraffins that originate downstream of the isomerization zone cannot be recycled to the isomerization zone without first having the non-paraffinic components removed. This again adds cost and complexity to the overall etherification process by requiring additional processing facilities.